reserve a,b,c for boolean object;
reserve p,q,r,s,A,B,C for Element of LTLB_WFF,
        F,G,X,Y for Subset of LTLB_WFF,
        i,j,k,n for Element of NAT,
        f,f1,f2,g for FinSequence of LTLB_WFF;

theorem Th4:
  for A holds A=TFALSUM or
  ex n st A=prop n or ex p,q st A=p=>q or ex p,q st A=p 'U' q
 proof
  let A;
  A=VERUM or A is simple or A is conjunctive or A is conditional by HILBERT2:9;
  hence thesis by HILBERT2:def 6,def 7,def 8;
 end;
